The present invention relates to a pneumatic tire, more particularly to a motorcycle tire provided with a tread reinforcing band suitable for running on rough terrains.
In general, a motorcycle tire for rough terrain is inflated to a tire pressure of 80 to 120 kPa when running on rough terrain, which pressure is relatively low in comparison with tire pressure of the on-road tires.
Therefore, the carcass of this kind of motorcycle tires is provided with a bias structure or a semi-radial structure comprising two cross plies of carcass cords in order to provide rigidity for the tire itself.
Conventionally, the tread portion of such bias or semi-radial tire is provided with a breaker comprising two cross plies of parallel cords laid at angles of from 20 to 50 degrees with respect to the tire circumferential direction.
In European patent application publication No. EP-1097824-A2, a motorcycle radial tire for on-road use is disclosed, wherein, instead of a type of the above-mentioned breaker, the tread portion is provided with a spiral band made of spirally wound steel cords. The steel cord is composed of waved steel filaments and non-waved steel filaments twisted together into a L×M structure (L, M: integers) such as 3×3 structure so that the cord has a variable tensile elastic modulus continuously increasing from its zero elongation to 1.0% elongation.
In the case of a carcass having a bias structure or semi-radial structure, so called vulcanization stretch of the carcass during tire vulcanization becomes large in comparison with a radial carcass structure. In other words, the outer diameter of the carcass at the tire equator becomes increased or the circumferential length of the carcass becomes increased more than the radial carcass structure.
As a result, if the above-mentioned spiral band is used in a tire having a carcass of a bias structure or semi-radial structure, then the elongation of the spiral band in the tire circumferential direction becomes insufficient for the vulcanization stretch, which results in defective molding or breakage of the band cord. Thus, it is difficult to manufacture the motorcycle tire acceptable in the marketplace.Otherwise, the hoop effect of the band becomes insufficient, and as a result, the rigidity of the tread portion can not be increased and good steering stability can not be obtained. Further, the high-speed durability is deteriorated.
A breaker can increase the tread rigidity totally, but it is inferior in the effect to increase the circumferential rigidity when compared with a spiral band.
If the circumferential rigidity of the tread portion is increased, then the traction of the tire during accelerating the motorcycle can be improved.
On the other hand, as disclosed in EP-1097824-A2, if waved steel filaments are used in order to achieve a special cord characteristic, the resultant steel cord has a tendency that the twist structure of the cord alone becomes unstable, and it becomes hard to deal with during building a raw tire.
Further, if a spiral band is incorporated in a motorcycle tire for rough terrain used under a relatively low tire pressure, since the band cord extends almost parallel with the tire circumferential direction, the band cord is subjected to a large bending deformation repeatedly during running and at the time of getting over protrusions.
Therefore, in comparison with a motorcycle tire for on-road use as disclosed in EP-1097824-A2, a larger fatigue resistance to bending deformation and a larger rupture strength are necessary.